The present invention relates to a photopolymerizable mixture which contains, as the essential constituents, (a) a compound, which is polymerizable by a free-radical mechanism and has terminal ethylenically unsaturated groups; (b) a polymeric binder; and (c) a polymerization initiator which can be activated by radiation.
Photopolymerizable mixtures of this type have been disclosed, for example, in U.S. Pat. Nos. 2,760,836, 3,060,023 and 3,149,975. The polymerizable, ethylenically unsaturated compounds described in these publications are low-molecular weight and high-molecular weight compounds having terminal or lateral vinyl groups or vinylidene groups, in particular acrylates and methacrylates of low-molecular weight or high-molecular weight polyhydroxy compounds. In practice, photopolymerizable materials based on esters of this type as the polymerizable compounds have gained acceptance almost exclusively. Among these, the low-molecular weight representatives in particular are preferentially used for industrial purposes.
During the processing of photopolymerizable mixtures of the type mentioned above it is necessary to eliminate, or at least to suppress as far as possible, the influence of the oxygen present in the ambient air on the radical polymerization, which in most cases is achieved by covering the photopolymerizable layer with a removable cover sheet which is impermeable to oxygen (U.S. Pat. No. 3,060,026) or with a cover layer which is impermeable to oxygen and is soluble in the developer used (U.S. Pat. No. 3,458,311). By this measure it is prevented that, during the light-induced polymerization, oxygen diffuses into the layer and causes chain termination reactions which result in an incomplete polymerization and thus in a reduced light sensitivity of the layer.
It is a disadvantage of these known materials that an additional working step is required for the application of the cover film or cover layer. In addition, the image resolution is reduced when self-supporting cover films, which in general have a thickness of about 20 .mu.m, are employed in contact copying processes. Cover layers which are soluble in developer lead to an increased developer consumption and to an additional pollution of the waste water. Therefore, attempts have been made to develop photopolymerizable materials which, even without a special cover layer, exhibit a sufficient light sensitivity in the imagewise polymerization.
DE-A No. 28 30 143 discloses a material of this type which contains specific, polymerizable compounds having a higher molecular weight, and specific initiator systems, and for this reason has a low sensitivity to oxygen. The polymerizable compounds employed are acrylates or methacrylates which have been obtained by reacting acrylic or methacrylic acid with bis-epoxy compounds.
Similar materials, which are preferentially employed for the production of photoresist images, are described in DE-A No. 26 02 410.
In addition to a number of advantages, these known materials have the disadvantage that only certain desired properties can be achieved by using them. For example, it is desired that photopolymerizable layers employed without an oxygen-inhibiting cover layer be non-tacky, but flexible and show no tendency to crystallize. In general, it is, however, impossible to achieve such a combination of properties without having to add further monomers which, in turn, have a negative effect on other advantageous properties.